This invention relates to use of phosphazenium salts as phase transfer catalysts. In one aspect the invention relates to a method of making aromatic ethers. More particularly, the method relates to a method of preparing aromatic ethers using exceptionally stable phosphazenium salt phase transfer catalysts.
Various types of aromatic ethers have gained prominence due to their utility in diverse fields as agricultural chemistry, medicinal chemistry and polymer chemistry. One class of aromatic ethers, aromatic polyethers (e.g. See for example polyethersulfones, polyetherimides, and polyetherketones), are important engineering resins due to their exceptional chemical and physical properties.
Aromatic ethers are typically prepared by synthetic methodology involving the reaction of the salt of an aromatic hydroxy compound with an aromatic compound comprising at least one suitable leaving group. In one general methodology, aromatic ethers are prepared in a nucleophilic aromatic substitution reaction between a nucelophilic aromatic hydroxy compound and an electrophonic aromatic compound comprising at least one suitable leaving group, the reaction being mediated by a stoichiometric amount of a basic reactant such as an alkali metal hydroxide or alkali metal carbonate. Typically, such nucleophilic aromatic substitution reactions must be carried out in polar aprotic solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidinone, dimethyl sulfoxide, or sulfolane in order to achieve synthetically useful rates of conversion of starting materials to product aromatic ethers. In such cases, drying, recovery, and reuse of the solvent is cumbersome and expensive.
Various phase transfer catalysts (PTC's) are known to accelerate reaction rates of chemical reactions generally. Phase transfer catalysts are typically most effective when the chemical reaction involves reactants which tend to segregate into separate phases. Among other benefits, the use of phase transfer catalysts is known to enable the use of solvents in which one or more of the reactants is insoluble in the absence of the phase transfer catalyst.
Known phase transfer catalysts include quaternary ammonium salts, quaternary phosphonium salts, and hexaalkylguanidinium salts. Of the known phase transfer catalysts, quaternary ammonium salts are stable at ambient temperature, but decompose rapidly at temperatures in excess of about 100° C. Quaternary phosphonium salts are more stable, but their use typically results in a lower reaction rate relative to the reaction rate observed in the corresponding reaction in which a quaternary ammonium salt phase transfer catalyst is employed. Thus, higher levels of phosphonium salt phase transfer catalyst must be used in order to achieve reaction rates comparable to reaction rates attained using quaternary ammonium salt phase transfer catalysts. Hexaalkylguanidinium salts are effective phase transfer catalysts but nonetheless are subject to decomposition at higher temperatures.
Much attention has been directed in recent years to organic reactions in heterogeneous systems, employing a phase transfer catalyst which facilitates the migration of a reactant into a phase from which it is normally absent. Many types of phase transfer catalysts are known to be effective under such conditions, including quaternary ammonium and phosphonium salts as disclosed in U.S. Pat. No. 4,273,712. Additionally, various bis-quaternary ammonium or phosphonium salts have been used as disclosed in U.S. Pat. No. 4,554,357; and aminopyridinium salts have been used as disclosed in U.S. Pat. Nos. 4,460,778, 4,513,141 and 4,681,949. Hexaalkylguanidinium salts, and their bis-salt analogues have been used as phase transfer catalysts as disclosed in U.S. Pat. Nos. 5,132,423; 5,116,975; and 5,081,298.
Nucleophilic aromatic substitution reactions, also referred to as “nucleophilic aromatic displacement reactions” often require heating a highly insoluble salt of an aromatic hydroxy compound with a soluble aromatic compound comprising at least one suitable leaving group in a relatively nonpolar solvent such as o-dichlorobenzene (o-DCB) in the presence of a phase transfer catalyst. Frequently, for synthetically useful reaction rates to be achieved, the reaction mixture must be heated to a temperature at which the phase transfer catalyst decomposes. While a prodigious technical effort has been expended in the development of more thermally stable phase transfer catalysts (See for example the development of 4-dialkylaminopyridinium salt catalysts and hexaalkylguanidinium salt catalysts), improved phase transfer catalyst thermal stability remains an important objective.
It would be highly desirable, therefore, to discover phase transfer catalysts having improved stability that could be used under a wide variety of reaction conditions, including the formation of aromatic ethers.